Reactance system



P 1953 K. s. STULL, JR 2,652,537

REACTANCE SYSTEM Filed NOV. 6, 1952 9+ 8+ PM i a II:

. J E 1/ J 9 I I: PRIOR 51 5+ 5+ FIG. 2 4

- TO osc. TANK FIG 5 TRANSCONDUCTANCE KEEFER S. STULL,JR.

INVENTOR.

BIAS OF GRID 28 Patented Sept. 15, 1953 UNITED STATES REACTANCE SYSTEM Keefer S. Stull, Jr., Baltimore, Md., assignor to Bendix Aviation Corporation, Towson, Md., a.

corporation of Delaware Application November 6, 1952, Serial No. 319,050

8 Claims.

This invention relates to systems for the control or variation of the frequency of an oscillator by the application and variation of a reactance in shunt with the tank circuit thereof.

The systems commonly used for this purpose utilize the plate current of an electron tube which is applied in shunt with the tank and so phased with the voltage across it as to have the effect or" a reactance. Variation of the magnitude of the plate current acts to vary the effective reactance of the tank circuit and so alter its resonant frequency.

Such systems possess two undesirable characteristics. In the first place, the tube has a static reactance which is always across the tank circuit when no control voltage is being applied. The application of a control voltage varies only the magnitude of the reactance about this static value. Secondly, due to the nature of the phase shift networks usually employed to control the phase of the grid voltage and to stray circuit el'e ments, the grid voltage of the reactance tube is not in exact phase quadrature with the tank voltage; i. e., there will be a component of grid voltage in phase with the tank voltage. This will produce an in-phase component of plate current which will appear resistive to the tank. This component is also varied by the control voltage and will therefore vary the loading on the tank to produce undesirable amplitude modulation of the oscillator when frequency modulation is attempted.

It is an object of this invention to provide a reactance system which has no static reactance and produces no variable loading of the tank circuit in response to control voltage variation.

It is a further object of the invention to provide a reactance system, the dynamic reactance of which varies in sign according to the instantaneous sign of the control voltage applied.

it is another object of the invention to provide a reactance which can be addedto an existing oscillator without changing its frequency calibration and without producing undesired amplitude modulation along with the frequency modulation.

These and other objects and advantages of the invention are realized by a system in which a reactance tube is provided with two grids acting as control grids; one having a 90 lagging R. F. voltage applied thereto, while a 90 leading R. F. voltage is applied to the other in relation to the plate voltage. The ratio of the amplitudes of these voltages is arranged to be the reciprocal of the ratio of the respective static gms of the two grids. In such an arrangement each grid will have an equal effect on the magnitude of the R. F. plate current and since they are excited in exact phase opposition the net R. F. plate current will be zero. The control voltage is thenapplied in such a way as to change the ratio of the gms. As an example, positive swings of the control voltage may be made to cause the cm of the lagging grid to increase in relation to that of the leading grid, giving thev tube a net inductive reactance, while on negative swings the reverse will happen and. the tube will have a net capacitive. reactance.

In the drawing:

Fig. 1 is a circuit diagram of a system illustrative of the prior art;

Fig. 2 is a circuit diagram of a reactance system embodying the invention; and,

Fig. 3 is a graph showing curves of cm vs. control voltage of a tube suitable for use in accordance with the invention.

The circuit shown in Fig. l is that of a typical reactance tube system for producing a frequency modulated signal or obtaining automatic frequency control. It is utilized in conjunction with an oscillator l having a resonant tank circuit 2-. The reactance tube is a pentode 3 having its anode coupled to the tank circuit by a capacitor :3 and its control grid connected by way of a capacitor 5 and resistor 6 in series. The control grid is also connected to ground through a capacitor l. A control voltage E is applied across terminals 8 and 9' to the control grid through an R. F. choke Hi. The control voltage may be audio frequency voltage for producing frequency modulation or direct voltage for use in automatic frequency control.

In the operation of the system of Fig. l, capacitor 5 is a blocking capacitor while capacitor 7 and resistor 6 connected across the tank 2 constitute a phase shift network. The resistance of t is made much larger than the capacitive reactance of 1; therefore the voltage across capacitor 'l, which is also the grid voltage of the reactance tube, lags the tank voltage by almost This causes the R. F. plate current of the reactance tube to lag the tank voltage by the same amount and since the tube i v in parallel with the tank circuit it appears as an inductive re" actance across the tank circuit thus affecting the resonant frequency of the oscillator. The R. F. plate current is approximately equal to the transconductance times the R. F. grid voltage, and the transconductance is a function of the grid bias; therefore the eiiective reactance of the tube can be varied by changing the grid bias. This is done by Varying the voltage E.

The circuit of Fig. l i subject to the defects pointed out above. It provides a static reactance across the tank circuit when no other control voltage E is applied, and the application of a control voltage varies only the magnitude or" the reactance about this static value. The grid voltage, furthermore, is not in exact quadrature with the tank voltage and will produce an in-phase component of plate current which will appear resistive to the tank.

These defects are avoided in the circuit. of Fig.

2. In that circuit a pentode 20 has its control grid 2! fed by a phase lag network consisting of an inductor 22 and a resistor 23 serially connected between anode and grid 2|, and a capacitor 24 between grid and ground. The capacitor 24 is shunted by the grid-to-ground stray capacitance indicated by the dotted line capacitor 25, and the series elements 22 and 23 are paralleled by the stray plate-grid capacitance indicated by 26. The cathode is connected to ground through bias resistors 36 and 37. The D. C. return for grid 2! is by way of resistor 21 to the junction of resistors 36 and 31. It is connected to the anode side of the phase shift network so that it will not contribute to the phase error.

The network consisting of resistor 23, and the capacitors 2 and 25 cannot alone produce quadrature phase shift because its elements have finite values and because of the stray capacitance 20. The addition of the coil 22 will, however, compensate the network so that exact quadrature phase shift will result, if

where the subscripts refer to the circuit elements identified by the corresponding reference characters.

The grid 28, normally utilized as the suppressor grid, is employed here as a second control grid and is excited through a phase lead network consisting of resistor 29, capacitor 30 and the stray plate-grid capacitance 3i. Resistor E'i is variable to provide a means for adjusting the bias on grid 20 to cause the circuit to present to the oscillator an exact zero static reactance. The stray grid-to-ground capacitance is represented by the dotted line capacitor 32. In order to obtain a phase shift of the nature desired the values of capacitor 30 and resistor 29 are so and if selected that the capacitive reactance presented by the capacitor 30 is large with respect to the resistance of resistor 29.

This network cannot produce exact quadrature phase shift because of the finite values of the resistance of resistor 29, the capacitance of capacitor 3i and because of the stray gridtoground capacitance 32. These phase errors can, however, be corrected by an inductor from grid-to-ground which can also be used as an R. F. choke to isolate the A. F. circuit. Such an inductor is the coil 33 which is R. F. grounded through capacitor dd. Perfect quadrature phase shift can be obtained with this network if X L E 2Z (ac+31) 32 The plate of tube as is coupled to the oscillator tank circuit by capacitor 38. Audio frequency voltage E is applied to control grid 28 at terminals 34, 35.

An ideal tube for use in a circuit such as shown in Fig. 2 is the type GASG, although the invention is not restricted to the use of this tube. When electrode voltages are properly chosen, the tube exhibits a basically linear characteristic over a limited range of bias in grid 28 as shown in Fig. 3. This chart shows that a variation of the bias on grid 28 causes the gm of both grids 2i and 28 I to vary approximately linearly but in opposite directions. Therefore, if both grids are excited by the R. F. quadrature voltages, and the control voltage is also superimposed on grid 28, the gm of both grids will be changed by the control voltage. In choosing the D. C. biases for grids 2| and 28 values should be selected which give the highest gm with good linearity. In general the bias on grid 2| should be as small as possible without exceeding the screen dissipation, while the bias on grid 28 should be set to fall in the middle of the linear region of the gm characteristics.

If both phase networks of the circuit of Fig. 2 are perfectly compensated, then the required relative gains of the networks are determined by the following relationship:

where 1721 and bzs are the absolute gains of the phase shift networks associated with the respective control grids; and

g'mzi and gmzs are the static values of gm for the respective grids.

The absolute values of gain should be made as high as possible, the actual limitations being the maximum grid swings permitted and the value of the R. F. plate voltage.

It can be shown that in the circuit of Fig 2. the effect of the tube on the oscillator tank circuit will be inductive when the instantaneous control voltage on grid 28 is positive relative to the bias voltage and capacitive when the control voltage is negative.

The following set of values for the components and parameters of the circuit of Fig. 2 is offered by way of example assuming a frequency of l-mc., an R. F. plate voltage of 30 volts, and a type of 6AS6 tube:

Plate and screen voltage- 120 volts D. C.

Bias on grid 21 1.43 volts. Bias on grid 28 2.0 volts. Resistor 3'6 120 ohms. Resistor 3'! ohms, variable (nominal value, 48 ohms). Peak value of control voltage 1.54 volts. Resistor 29 470 ohms. Capacitance 3| 1.7 mmf. (assumed). Capacitor 30 3.9 mmf. Inductor 33 2.1 mh. Capacitance 32 6.4 mmf. (assumed). Capacitance 20 1.0 mmf. (assumed). Resistor 23 68,000 ohms. Inductor 22 19.2 mh. Capacitor 24 68 mmf. Capacitance 25 6.5 mmf. (assumed). Resistor 2'1 1,000,000 ohms.

These values will produce a frequency deviation of 5.05 kc. per volt of control voltage.

applied to any electrode or combination of electrodes which is effective to vary the space current'flow of the tube.

What is claimed is:

l. A reactance system comprising an electron tube having an anode, a cathode and at least two grid electrodes, a pair of phase shifting networks, each connecting said anode with a respective one of said grid electrodes, one of said networks advancing the phase of the voltage at the grid electrode to which it is connected by ninety degrees with respect to the voltage at said anode, the other of said networks delaying the phase of the voltage at the grid electrode to which it is connected by ninety degrees with respect to the voltage at said anode, means biasing said grid electrodes such that the ratio of the static transconductances of said grid electrodes is the reciprocal of the ratio of the amplitudes of the voltages applied thereto by said networks, means applying a control voltage to one of the electrodes of said tube and an output connection to said anode.

2. A reactance system comprising an electron tube having an anode, a cathode, a control grid and a suppressor grid, a pair of phase shifting networks, each connecting said anode with a respective one of said grids, one of said networks advancing the phase of the voltage at the grid to which it is connected by ninety degrees with respect to the voltage at said anode, the other of said networks delaying the phase of the voltage at the grid to which it is connected by ninety degrees with respect to the voltage at said anode, means biasing said grids such that the ratio of the static transconductances of said grids is the reciprocal of the ratio of the amplitudes of the voltages applied thereto by said networks, means applying a control voltage to one of said grids and an output connection to said anode.

3. A reactance system comprising an electron tube having an anode, a cathode, a control grid and another grid, a phase shifting network connecting said anode and said control grid and delaying the phase of the voltage at said control grid by ninety degrees with respect to the voltage at said anode, a phase shifting network connecting said anode and said other grid and advancing the voltage at said other grid by ninety degrees with respect to the voltage at said anode, means biasing said grids such that the ratio of the static transconductances of said grids is the reciprocal of the ratio of the amplitudes of the voltages applied thereto by said networks, means applying a control voltage to one of said grids and an output connection to said anode.

4. A reactance system comprising an electron tube having an anode, a cathode, a control grid and another grid, a phase shifting network connecting said anode and said control grid and de laying the phase of the voltage at said control grid by ninety degrees with respect to the voltageat said anode, a phase shifting network connecting said anode and said other grid and advancing the phase of the voltage at said other grid by ninety degrees with respect to the voltage at said anode, means biasing said grids such that the ratio of the static transconductances of said grids is the reciprocal of the ratio of the amplitudes of the voltages applied thereto by said networks, means applying a control voltage to said other grid and an output connection to said anode.

5. In combination with an oscillator having a resonant tank circuit, means for controlling the frequency of said oscillator comprising an electron tube having its space discharge path connected across said tank circuit, said tube having an anode and at least two grid electrodes, a pair of phase shifting networks, each connected between said anode an a respective one of said grid electrodes, one of said networks delaying the phase of the voltage at the grid electrode to which it is connected by ninety degrees with respect to the voltage at said anode, the other of said networks advancing the phase of the grid electrode to which it is connected by ninety degrees with respect to the phase of the voltage at said anode, means biasing said grid electrodes such that the ratio of the static transconductances of said grid electrodes is the reciprocal of the ratio of the amplitudes of the voltages applied thereto by said networks and means applying a control voltage to one of the electrodes of said tube.

6. In combination with an oscillator having a resonant tank circuit, means for controlling the frequency of said oscillator comprising an electron tube having its space discharge path connected across said tank circuit, said tube having an anode, a control grid and a suppressor grid, a pair of phase shifting networks, each connecting said anode with a respective one of said grids, one of said networks advancing the phase of the voltage at the grid to which it is connected by ninety degrees with respect to the voltage at said anode, the other of said networks retarding the phase of the voltage at the grid to which it is connected by ninety degrees with respect to the voltage at said anode, means biasing said grids such that the ratio of the static transconductances of said grids is the reciprocal of the ratio of the amplitudes of the voltages applied thereto by said networks and means applying a control voltage to one of said grids.

7. In combination with an oscillator having a resonant tank circuit, means for controlling the frequency of said oscillator comprising an electron tube having its space discharge path connected across said tank circuit, said tube having an anode, a control grid and another grid, a phase shifting network connecting said anode and said control grid and delaying the voltage at said control grid by ninety degrees with respect to the voltage at said anode, a phase shifting network connecting said anode and said other grid and advancing the phase of the voltage at said other grid by ninety degrees with respect to the voltage at said anode, means biasing said grids such that the ratio of the static transconductances of said grids is the reciprocal of the ratio of the amplitudes of the voltages applied thereto by said networks and means applying a control voltage to one of said grids.

8. In combination with an oscillator having a resonant tank circuit, means for controlling the frequency of said oscillator comprising an electron tube having its space discharge path connected across said tank circuit, said tube having an anode, a control grid and another grid, a phase shifting network connecting said anode and said control grid and delaying the voltage at said control grid by ninety degrees with respect to the voltage at said anode, a phase shifting network connecting said anode and said other grid and advancing the phase of the voltage at said other grid by ninety degrees with respect to the voltage at said anode, means biasing said grids such that the ratio of the static transconductances of said grids is the reciprocal of the ratio of the amplitudes of the voltages applied thereto by said networks and means applying a control voltage to said other grid.

KEEFER S. STULL, JR.

No references cited. 

